Mammalian spermatogenesis is composed of a series of complicated cytological events which are controlled by the microenvironment within the testis. There have been many studies dealing with the hormonal control of spermatogenesis. Despite this, the effect of specific hormones on specific steps of spermatogenesis remains largely unknown primarily because we have lacked techniques to assess the hormonal milieu in the intratesticular microenvironment and to quantify the myriad of cytological events and cell-cell interactions involved. In this application, it is proposed to test the effect of testosterone (T) and dihydrotestosterone (DHT) in the seminiferous tubular fluid (STF) and interstitial fluid (IF) on cytological events in the seminiferous tubule. This is possible because of the recent development of a centrifugation technique which enables us to systematically collect intratesticular fluid samples for steroid hormone assay and recent advances in stereological techniques for reliable assessment of morphological quantities. Two treatments, hypophysectomy and implantation of testosterone-estradiol capsules, wil be used to obtain animals with low levels of T and DHT in STF and IF, and with regressed spermatogenesis. Then various quantities of exogenous T will be supplied to these animals via Silastic implants either to maintain or to re-initiate spermatogenesis. It is planned to assess via stereological techniques at the light and electron microscopic levels changes in the seminiferous epithelium and Sertoli cells. These changes will be correlated with the cycle of the seminiferous epithelium and with the T and DHT concentrations in IF and STF. This proposal is unique in two ways: 1) stereological principles, which make possible the integration of three-dimensional information from two-dimensional images, will be utilized to reveal changes in specific spermatogenic cytological events, and 2) intratesticular androgen changes accompanying the change in spermatogenesis will be systematically evaluated. Results from the proposed experiments will fill the gap of previous studies on the regulation of mammalian spermatogenesis. Our knowledge concerning how to carry out infertility treatment and fertility control will be significantly increased.